The present invention relates to a polarization converter that efficiently converts substantially normally incident unpolarized light into linearly polarized light. More specifically, the present invention relates to a relatively thin lenticular polarization converter and to liquid crystal display (LCD) projection system designs including the novel thin polarization converter, for example, full-color LCD projection systems.
Polarization dependent spatial light modulators, such as some LCD devices, require polarized light. Two challenges in designing polarization converters for use with LCD devices are compactness and efficiency. A compact planar polarization device that efficiently converts unpolarized light to polarized light would aid greatly in the design of compact and portable LCD devices.
Unpolarized light may be decomposed into a linear s-polarization component and an orthogonal p-polarization component. A method for producing polarized light for a LCD projection panel comprises the use of a polarizing beam splitter (PBS) cube or rectangular prism. One linearly polarized component of the light is transmitted by the PBS cube and directed to the LCD panel, while the orthogonal component is reflected away in a perpendicular direction. Other devices use non-cubic polarization splitters (non-rectangular prisms). Both the cubic and the non-cubic devices have a considerable longitudinal dimension (in the direction of travel of light) in relation to its transverse dimension. (An exemplary device has thickness dimensions that are approximately one fourth of the width dimensions.)
Another common method for producing polarized light comprises the use of an absorbing dye or iodine based polarizer film positioned between the light source and the LCD panel. The absorbing film transmits a single component linear polarized light, while absorbing the orthogonal component. Accordingly, the maximum conversion efficiency that may be achieved with an absorbing polarizer is 50% or less. The absorbing polarizer film is often integrally incorporated into the commercial LCD panel. Alternatively, a separate polarizer plate may be positioned between the light source and LCD.
Both a plain PBS cube and an absorbing polarizer are inefficient, in that a maximum of only one-half of the available light from the source is converted to polarized light for transmission through the LCD panel. Attempts have been made to recycle the reflected polarization component from a PBS cube. However, solid glass PBS cubes are bulky and impractical for applications in which the diagonal of the spatial light modulator exceeds approximately 50 mm.
Some existing polarization converters include lenslet arrays followed by a polarizing component. The lenslet array commonly includes an array of "Galilean telescopes", that is, the first surface of the array has convex lenslets that focus the light and the second surface has concave lenslets to recollimate the light. The lenslets on the second surface are smaller than those on the first so the intervening spaces can be used to convert the light from one polarization state to another. The component that converts the polarization state is always in the light path following the recollimating lenslets.
Recently, reflective polarizing sheet films have been developed. Use of a reflective polarizing sheet film, instead of an absorbing sheet polarizer, allows for the possibility of reflecting back the s-polarization component of a light beam in the direction of the light source. Methods have been described that return the reflected polarized light to a reflector behind the light source, and back to the LCD panel. However, these methods require extremely precise alignment of the optical components for efficient recycling of the light and are not easily suitable for compact applications.
FIG. 1 illustrates a polarization converter 10 depicted in U.S. Pat. No. 5,566,367. A beam of incident unpolarized and collimated light 70 is compressed into collimated sub-beams 72 by a lenticular element 20. The lenticular element includes an entrance surface 22 and an exit surface 30. The entrance surface is comprised of converging lenslets 24, while the exit surface has diverging lenslets 34. The resulting sub-beams 72 are incident upon a prismatic element 40. Linearly polarized beams 74 exit the prismatic element 40. The prismatic element 40 includes entrance side prisms 42, a series of quarter-wave retarder films 44, and exit side prisms 46. The exit side prisms 46 have reflective polarization beam splitting coatings 50 on one of their faces and total reflection mirrors 52 on another. As may be appreciated by the light paths described, this polarization converter design requires precise thickness control and precise registration between the elements. The polarization converter further requires collimated light. The deposition of the required coatings on selective prismatic surfaces presents significant manufacturing challenges, as does the assembly of the prisms with retarder films.
Other systems attempt to improve efficiency by recycling the reflected polarized light from various types of polarization producing films without returning the light to the light source. Some of these systems use polarization conversion devices that use holographic optical elements to separate the polarization components. All of these systems can take up considerable space and are not suitable for compact applications or for large-gate LCD panels.